U.S. patent application number 16/605239 was filed with the patent office on 2020-04-23 for light escalators in optical circuits between thick and thin waveguides.
The applicant listed for this patent is Teknologian tutkimuskeskus VTT Oy. Invention is credited to Timo Aalto, Sanna Arpiainen, Matteo Cherchi.
Application Number | 20200124795 16/605239 |
Document ID | / |
Family ID | 63855623 |
Filed Date | 2020-04-23 |
United States Patent
Application |
20200124795 |
Kind Code |
A1 |
Cherchi; Matteo ; et
al. |
April 23, 2020 |
Light escalators in optical circuits between thick and thin
waveguides
Abstract
The invention relates to photonic circuits, in particular to
photonic circuits where light is escalated transferred between
optical waveguides which are coupled to photonic devices. A first
waveguide on a silicon substrate is provided having a first
thickness and a first refractive index. A tapered second waveguide
having a second thickness less than the first thickness and a
second refractive index higher than said first refractive index is
deposited on the first waveguide. At least one layer of an
optically active material comprising a photonic device is deposited
on the first waveguide adjacent to the second waveguide. The
photonic device is interfaced with the wide end of the tapered
second waveguide to provide an optical coupling, and the opposite
narrow end of the tapered second waveguide is interfaced on top of
the first waveguide to provide adiabatic light transfer between
said first and second waveguides.
Inventors: |
Cherchi; Matteo; (VTT,
FI) ; Aalto; Timo; (VTT, FI) ; Arpiainen;
Sanna; (VTT, FI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Teknologian tutkimuskeskus VTT Oy |
Espoo |
|
FI |
|
|
Family ID: |
63855623 |
Appl. No.: |
16/605239 |
Filed: |
April 23, 2018 |
PCT Filed: |
April 23, 2018 |
PCT NO: |
PCT/FI2018/050287 |
371 Date: |
October 15, 2019 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62488101 |
Apr 21, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/14 20130101; G02B
6/1228 20130101; G02B 2006/12152 20130101; G02B 2006/12061
20130101; G02B 2006/12035 20130101; G02B 6/125 20130101; G02B
2006/12138 20130101; G02B 2006/12147 20130101; G02B 2006/12142
20130101 |
International
Class: |
G02B 6/122 20060101
G02B006/122 |
Claims
1. A photonic circuit, comprising: a first waveguide on a silicon
substrate, said waveguide having a thickness of 1-12 .mu.m and a
refractive index of 3-3.5; an second waveguide comprising amorphous
silicon and having a thickness of 0.1-1 .mu.m and a refractive
index of 3.1-4, said waveguide having a tapered shape with a
cross-section that is smaller at one end and larger at the opposite
end at least in one direction; and at least one layer of an optical
material arranged to optically interface with said second
waveguide, said layer comprising a photonic device; wherein said
end of said tapered second waveguide having a smaller cross-section
is interfaced with said first waveguide to provide adiabatic light
transfer between said first and second waveguides, and wherein said
photonic device is interfaced with said end of said second
waveguide having a larger cross-section to provide optical coupling
between said second waveguide and said photonic device.
2. The photonic circuit according to claim 1, wherein said at least
one second waveguide is deposited on said first waveguide.
3. The photonic circuit according to claim 1, wherein said at least
one second waveguide at least partially overlaps said photonic
device.
4. The photonic circuit according to claim 1, wherein said photonic
device comprises: at least one layer of an optical material
deposited on said first waveguide; a layer of a dielectric material
deposited on each layer of said optical material; and said second
waveguide deposited partly on the uppermost dielectric layer and
partly on said first waveguide, whereby said photonic device is
interfaced with said second waveguide to provide an optical
coupling at said larger end of said second waveguide, and said
second waveguide is interfaced with said first waveguide to provide
adiabatic light transfer between said first waveguide and said
second waveguide at said smaller end of said optical waveguide.
5. The photonic circuit according to claim 1, wherein the photonic
circuit comprises an etch-stop layer between at least one of said
first waveguide, said second waveguide and said at least one layer
of said optical material.
6. The photonic circuit according to claim 1, wherein said first
waveguide is a strip waveguide made of crystalline silicon.
7. The photonic circuit according to claim 1, wherein said second
waveguide is a waveguide made of amorphous silicon or hydrogenated
amorphous silicon.
8. The photonic circuit according to claim 1, wherein said
etch-stop layer comprise silica, silicon nitride or thermally
oxidized silicon dioxide SiO.sub.2.
9. The photonic circuit according to claim 1, wherein said optical
material comprises at least one layer of graphene, germanium or a
silicon-germanium alloy.
10. The photonic circuit according to claim 4, wherein in the
layers of said dielectric material is etched openings to provide
electrical contact to the layers of said at least one of said
optical material layers from contact terminals patterned on said
second waveguide.
11. The photonic circuit according to claim 4, wherein said
dielectric material comprises aluminum oxide, silicon nitride or
silicon dioxide.
12. The photonic circuit according to claim 1, wherein said second
waveguide is tapered in the horizontal plane of said substrate to
provide cross-sections which is smaller at one end and larger at an
opposite end.
13. The photonic circuit according to claim 1, wherein said second
waveguide is tapered in the vertical plane of said substrate to
provide cross-sections which is smaller at one end and larger at an
opposite end.
14. A light detector circuit, comprising: a first waveguide on a
silicon substrate, said waveguide having a thickness of 1-12 .mu.m
and a refractive index of 3-3.5; an etch-stop layer on said first
waveguide; at least one second waveguide deposited on said first
waveguide and said etch-stop layer comprising amorphous silicon and
having a thickness of 0.1-1 .mu.m and a refractive index of 3.1-4,
said second waveguide having a tapered cross-section that is
smaller at one end and larger at the opposite end at least in one
direction; and a layer of germanium deposited on said first
waveguide adjacent to or partially overlapping with said at least
one second waveguide, said germanium layer comprising a light
detector; wherein said light detector is interfaced with said end
of said tapered second waveguide having a larger cross-section to
provide an optical coupling, and the opposite end of said tapered
second waveguide having a smaller cross-section is interfaced on
top of said first waveguide to provide adiabatic light transfer
between said first and second waveguides.
15. The light detector circuit according to claim 14, wherein said
first waveguide is a strip waveguide made of crystalline
silicon.
16. The light detector circuit according to claim 14, wherein said
second waveguide is a waveguide made of amorphous silicon or
hydrogenated amorphous silicon.
17. The light detector circuit according to claim 14, wherein said
etch-stop layer comprises silica, silicon nitride or thermally
oxidized silicon dioxide SiO.sub.2.
18. A modulator circuit, comprising: a first waveguide on a silicon
substrate, said waveguide having a thickness of 1-12 .mu.m and a
refractive index of 3-3.5; an etch-stop layer on said first
waveguide; two layers of graphene deposited on said first waveguide
and said etch-stop layer, a layer of a dielectric material
deposited on each layer of graphene, said graphene and dielectric
layers comprising a modulator; and at least one second waveguide
deposited on the uppermost dielectric layer comprising amorphous
silicon and having a thickness of 0.1-1 .mu.m and a refractive
index of 3.1-4, said second waveguide having a tapered
cross-section that is smaller at one end and larger at the opposite
end at least in one direction; wherein said modulator is interfaced
between said first and second waveguides to provide an optical
coupling to said second waveguide, and said end of said tapered
second waveguide having a smaller cross-section is interfaced on
top of said first waveguide to provide adiabatic light transfer
between said first and second waveguides.
19. The modulator circuit according to claim 18, wherein said first
waveguide is a strip waveguide made of crystalline silicon.
20. The modulator circuit according to claim 18, wherein said
second waveguide is a waveguide made of amorphous silicon or
hydrogenated amorphous silicon.
21. The modulator circuit according to claim 18, wherein said
etch-stop layer comprises silica, silicon nitride or thermally
oxidized silicon dioxide SiO.sub.2.
22. The modulator circuit according to claim 18, wherein said
dielectric material comprises aluminum oxide, silicon nitride or
silicon dioxide.
Description
FIELD OF THE INVENTION
[0001] The invention relates to photonic circuits and their
manufacture, in particular to photonic circuits where light is
transferred between optical waveguides, which are coupled to
photonic devices.
BACKGROUND OF THE INVENTION
[0002] Optical communication systems are continuously being
miniaturized to integrate a large number of previously discrete
optoelectronic devices with silicon-based integrated circuits to
achieve on-chip optical interconnects for high performance
computation. In particular, silicon photonics aims to integrate as
many as possible optoelectronic functionalities based on CMOS
compatible materials, in order to lower the cost without
sacrificing performance.
[0003] Optical modulators and photodetectors are main building
blocks of photonic systems. These two types of devices operate
based on very different mechanisms and consequently utilize
different device geometries. They often have to be made of
different materials that are difficult and costly to integrate with
silicon photonics. Optical modulators are based on electro-optical
or electro-absorptive effects in materials such as LiNbO.sub.3,
germanium and compound semiconductor heterostructures. In silicon
photonics, the dispersion effect induced by carrier injection or
depletion is the most common method used to achieve integrated
optical modulation, both in amplitude and phase. Typically this
requires several millimeter long devices, but amplitude modulation
can be also achieved with micron-scale devices, based on the
Franz-Keldysh effect in SiGe compounds.
[0004] At the receiving end of optical links, photodetectors
convert light back into electrical signals by absorbing photons and
generating charges through photo-electric effects. Therefore,
strong absorption and effective collection of photo-excited
carriers are desired for efficient photo-detection. Because of
these distinctive requirements, to date no device that can function
as both a photodetector and a modulator, and whose role can be
switched through external control, has been made with a single type
of material. Such a simple yet multifunctional device, if
implemented, not only can make integrated optical systems
programmable and adaptable, but also can lead to novel applications
such as optoelectronic oscillators and new schemes of optical
computation and signal processing.
[0005] Because of its two-dimensional structure, graphene is
ideally suited for integration with planar photonic devices and the
performance of the devices benefits significantly from the
elongated optical interaction length in coplanar configuration.
With its remarkable optical and electrical properties, including
absorption and dispersion, graphene has been exploited as a
multifunctional optoelectronic material to produce, for example,
highly tunable optoelectronic devices with high performance and
adaptive controllability by electrostatic gating or chemical
doping. Such devices include photodetectors, optical modulators,
polarizers and saturable absorbers. Graphene optical modulators
have been demonstrated to have very high speed (to date only
limited by the RC constant of the electrodes) and very low energy
consumption.
[0006] Indeed, graphene has been demonstrated to perform as a
modulator on thin SOI waveguides with theoretical modulation speed
of 800 GHz which would be far beyond other technological platforms
used e.g. in optical switches in data centres. The problem to be
solved is how to structure and fabricate graphene modulators on
thick SOI waveguides, so that the graphene is interacting with the
optical field.
[0007] Photonics circuits based on micron-scale (3 .mu.m thick)
silicon-on-insulator (SOI) waveguides have many advantages compared
to standard submicron (220 nm-400 nm thick) silicon waveguide
technology, but also some major limitations. Namely larger
waveguide size implies devices with larger power consumption and,
even more importantly, slower speed. This makes this thicker
platform less attractive compared to others for most applications
requiring high speed modulation and detection.
[0008] Prior art solutions to the problem include the use of
submicron silicon waveguides for the whole circuit, which comes
with many drawbacks, including high propagation losses,
single-polarization operation, bad tolerances to fabrication
errors, not to mention the requirement for very expensive deep UV
fabrication tools. Fast devices have been demonstrated on thick SOI
waveguides, where the volume of the devices was reduced by
patterning sub-micron wide waveguides with a high aspect ratio,
which makes the fabrication challenging and with a bad impact on
yield. Only some types of fast detectors and modulators have been
demonstrated with this approach, and no broadband modulators or
phase modulators.
[0009] It is an object of the present invention to create an
interface between micro-scale waveguides and submicron waveguides,
in order to exploit the advantages of both technologies. One of the
advantages of submicron waveguides is that they interact much
better with graphene layers. Important other advantages include the
possibility to fabricate fast Ge detectors and SiGe Franz-Keldysh
modulators.
[0010] The present invention aims to make the micron-scale SOI
platforms more attractive in the implementation of photonic
circuits, by changing the mode size into the submicron scale only
when fast modulation and detection is needed, while keeping the
advantages of micron-scale waveguides elsewhere
SUMMARY OF THE INVENTION
[0011] According to one aspect of the invention, a photonic circuit
is provided, comprising: [0012] a first waveguide on a silicon
substrate, said waveguide having a thickness of 1-12 .mu.m and a
refractive index of 3-3.5; [0013] an second waveguide comprising
amorphous silicon and having a thickness of 0.1-1 .mu.m and a
refractive index of 3.1-4, said waveguide having a tapered shape
with a cross-section that is smaller at one end and larger at the
opposite end at least in one direction; [0014] at least one layer
of an optical material arranged to optically interface with said
second waveguide, said layer comprising a photonic device; wherein
[0015] said end of said tapered second waveguide having a smaller
cross-section is interfaced with said first waveguide to provide
adiabatic light transfer between said first and second waveguides,
and wherein said photonic device is interfaced with said end of
said second waveguide having a larger cross-section to provide
optical coupling between said second waveguide and said photonic
device.
[0016] The invention thus concerns a light escalator concept, where
the light is moved from a thick initial HIC waveguide (e.g. made of
Si) to a thin HIC waveguide deposited on top and which is made of a
low-loss material with higher refractive index than normal silicon,
e.g. a-Si:H alloy. The thin waveguide it at one end coupled to a
device that may be made of a different material and is deposited or
otherwise added on top of the thick HIC waveguide.
[0017] Within the inventive concept, various refractive index
ranges and waveguide size ranges may be specified for the initial
(e.g. Si) and the final (e.g. a-Si:H) waveguide.
[0018] According to further aspects of the invention, photonic
circuits, such as an inventive light detector circuit comprises:
[0019] a first waveguide on a silicon substrate, said waveguide
having a thickness of 1-12 .mu.m and a refractive index of 3-3.5;
[0020] an etch-stop layer on said first waveguide; [0021] at least
one second waveguide deposited on said first waveguide and said
etch-stop layer comprising amorphous silicon and having a thickness
of 0.1-1 .mu.m and a refractive index of 3.1-4, said second
waveguide having a tapered cross-section that is smaller at one end
and larger at the opposite end at least in one direction; [0022] a
layer of germanium deposited on said first waveguide adjacent to or
partially overlapping with said at least one second waveguide, said
germanium layer comprising a light detector; wherein [0023] said
light detector is interfaced with said end of said tapered second
waveguide having a larger cross-section to provide an optical
coupling, and the opposite end of said tapered second waveguide
having a smaller cross-section is interfaced on top of said first
waveguide to provide adiabatic light transfer between said first
and second waveguides.
[0024] Further, according to some embodiments, an inventive
modulator circuit may comprise: [0025] a first waveguide on a
silicon substrate, said waveguide having a thickness of 1-12 .mu.m
and a refractive index of 3-3.5; [0026] an etch-stop layer on said
first waveguide; [0027] two layers of graphene deposited on said
first waveguide and said etch-stop layer, [0028] a layer of a
dielectric material deposited on each layer of graphene, said
graphene and dielectric layers comprising a modulator; [0029] at
least one second waveguide deposited on the uppermost dielectric
layer comprising amorphous silicon and having a thickness of 0.1-1
.mu.m and a refractive index of 3.1-4, said second waveguide having
a tapered cross-section that is smaller at one end and larger at
the opposite end at least in one direction; wherein said modulator
is interfaced between said first and second waveguides to provide
an optical coupling to said second waveguide, and said end of said
tapered second waveguide having a smaller cross-section is
interfaced on top of said first waveguide to provide adiabatic
light transfer between said first and second waveguides.
[0030] An inventive modulator may be interfaced between said first
and second waveguides to provide an optical coupling to the thinner
second waveguide, and the tapered end of the thin waveguide having
a smaller cross-section is interfaced on top of the thick waveguide
to provide adiabatic light transfer between the first and second
waveguides.
[0031] Both the thick and the thin waveguide are high index
contrast (HIC) waveguides with the refractive index of the core
being at least 1 refractive index unit larger than the refractive
index of the surrounding cladding materials (excluding the other
waveguide core and photonic device materials). The thick waveguide
can be made of crystalline silicon, for example. Underneath the
thin second waveguide, the thick first waveguide is horizontally
patterned into a horizontally confined waveguide. The thin
waveguide can be made of a material comprising amorphous
silicon.
[0032] According to some embodiments, the second waveguide is
deposited on the first waveguide to partially overlap the photonic
device. The material having a second refractive index may be
amorphous silicon or hydrogenated amorphous silicon, which is
partially overlapping an optical material, such as graphene.
[0033] According to some embodiments, the second waveguide acts as
an intermediate waveguide, through which light is coupled into a
third waveguide that is placed on top of the second waveguide. In
this case light is adiabatically coupled from the second waveguide
to the third waveguide, which forms the optical device. The third
waveguide can be formed, for example, in a 200-500 nm thick layer
of crystalline silicon or in a layer of III-V compound
semiconductor material added on top of the second waveguide.
[0034] In some embodiments of the invention, the first waveguide is
covered with an etch-stop layer prior to depositing the second
waveguide and/or said at least one layer of an optical
material.
[0035] In some embodiments of the invention, the first waveguide is
a strip waveguide made of crystalline silicon. In some embodiments
of the invention, the second waveguide is made of amorphous silicon
or hydrogenated amorphous silicon. The exact properties of the
material naturally depend on the concentrations of any
participating materials, such as germanium.
[0036] In some embodiments, the dielectric material may comprise
aluminum oxide, silicon nitride or silicon dioxide, for example.
The etch-stop layer may comprise silica, silicon nitride or
thermally oxidized silicon dioxide SiO.sub.2, and the dielectric
material comprise aluminum oxide, silicon nitride or silicon
dioxide.
[0037] According to some embodiments of the invention, the optical
material of the photonic device may comprise at least one layer of
graphene, germanium or a silicon-germanium alloy.
[0038] In some embodiments of the invention, the contact to the
layers of said at least one optical material layers are formed
through etched openings in the dielectric material layers to enable
contact to contact terminals patterned on the optical
waveguide.
[0039] According to some embodiments, the thinner second waveguide
is formed to a tapered shape having a cross-section in the
horizontal plane of said substrate which is smaller at one end and
larger at an opposite end of said tapered waveguide. In other
embodiments, the second waveguide is formed to a tapered shape
having a cross-section in the vertical plane of the substrate,
which is smaller at one end and larger at an opposite end of said
tapered waveguide. Obviously, the invention is not restricted to
tapered shapes only, as long as the cross section is smaller at one
end and larger at the opposite end.
[0040] The invention offers considerable benefits. In the case of
detectors only a thin layer of Ge is needed, instead of the 3 .mu.m
thick Ge layer usually needed to make a detector on a 3 .mu.m thick
SOI. In this way the detector volume can be small and capacitance
of vertical contacts can be low, paving the way to high speed
devices. Also, unlike a 3 .mu.m thick germanium layer grown on
etched silicon, in the present invention Ge may be grown on high
quality non-etched silicon surface, which makes the quality of the
material much higher, with positive impact on sensitivity and dark
current. Further, contacts for the devices can be implanted
directly on flat a thick silicon surface. Similarly, a thin layer
of SiGe alloy with suitable bandgap can be deposited to realize
fast Franz-Keldysh modulators with smaller volume than existing
devices in microscale silicon platforms. 2D materials, such as
graphene, can be easily integrated and sandwiched between
waveguides of amorphous and crystalline silicon, with suitable
dielectric insulating layers.
[0041] Photonic circuits built according to the present invention
are potentially much faster than present modulators based on thick
SOI waveguides.
[0042] The inventive technology may be used and applied in
monolithic integration of thin and thick waveguides, i.e. the
possibility to fabricate thick and thin waveguides within the same
fabrication process, making available a platform with the
advantages of both types of waveguides. Integrated optics is an
enabling technology, with a long list of possible applications,
from integrated optical modulators and photodetectors for high
speed optical switching in telecommunications data centres to gas
sensing, and from medical imaging to LIDAR systems.
Definitions
SOI--Silicon On Insulator
[0043] a-Si--Amorphous silicon a-Si:H--Hydrogenated amorphous
silicon poly-Si--Polycrystalline silicon a-SiGe:H--amorphous
silicon-germanium SiGe--silicon-germanium alloy Franz-Keldysh
modulator--an electro-absorption modulator for controlling the
intensity of a laser light via an electric voltage based on the
Franz-Keldysh effect, i.e. a change in the absorption spectrum
caused by an applied electric field changes the bandgap energy
optical material--a material consisting of, for example, graphene,
germanium or a silicon-germanium alloy, which can be optically
active, i.e. form a controllable photonic device, i.e. a modulator
thick waveguide--a waveguide having a thickness of 1-12 .mu.m, and
a refractive index in the range of 3-3.5. The waveguide may consist
of, for example, crystalline silicon, indium phosphide, gallium
arsenide, or any other high-refractive index transparent material
that receives input light from one direction and may feed an
optical waveguide with that light in another direction. The
material may be designed to be a waveguide in itself. thin
waveguide--a submicron scale low-loss waveguide having a thickness
of 0.1-1 .mu.m and a refractive index in the range of 3.1-4. The
waveguide consists of amorphous silicon or hydrogenated amorphous
silicon.
BRIEF DESCRIPTION OF THE DRAWINGS
[0044] FIG. 1 illustrates the phenomenon of adiabatic light
transfer;
[0045] FIG. 2 shows in a side view and a top view the light
transfer in an exemplary inventive waveguide;
[0046] FIGS. 3A and 3B shows side and top views of a photonic
device according to one embodiment of the invention;
[0047] FIGS. 4A and 4B shows side and top views of a photonic
device according to another embodiment of the invention;
[0048] FIGS. 5A and 5B shows side and top views of a photonic
device according to a further embodiment of the invention;
[0049] FIGS. 6A and 6B shows an embodiment of the invention with
high index contrast waveguides with a silica bottom cladding;
[0050] FIGS. 7A and 7B shows a further embodiment of high index
contrast waveguides with a silica bottom cladding.
[0051] FIG. 8 In shows the path of a light beam passing through an
inventive device like one in FIG. 7A or 7B.
DESCRIPTION OF EMBODIMENTS
[0052] FIG. 1 illustrate the phenomenon of adiabatic light
transfer. In FIG. 1 is shown a schematic top view a system of three
waveguides consisting of two circularly bent outermost waveguides L
and R, and one straight central waveguide C. The minimum distance
between waveguides is given by x.sub.0, and the z distance between
the centers of the curved waveguides is defined by .delta.. The
radius of curvature of the outermost waveguides L, R may be is 3.5
m, the spatial delay .delta.=4200 .mu.m, for example, and the
minimum separation between waveguides x.sub.0=7 .mu.m, for
example.
[0053] FIG. 2 shows a side view and a top view of an exemplary
inventive waveguide, where a simulation shows how the light is
transferred from a lower thick silicon waveguide 20 made of
crystalline silicon Si to an upper tapered (22) and thin
hydrogenated amorphous silicon (a-Si:H) waveguide 21, with a
refractive index higher than that of crystalline silicon. The
cross-section "a" to the left further illustrate the light
distribution at the left end of the waveguide and the cross-section
"b" to the right the same situation at the right end.
[0054] FIGS. 3A and 3B show an inventive photonic device 30
comprising a thin submicron (t.sub.2=about 500 nm thick, for
example) low-loss waveguide made of a hydrogenated amorphous
silicon (a-Si:H) layer 32 having a refractive index higher than
that of crystalline silicon. The waveguide 32 is deposited on top
of a thick Si strip waveguide 33a, which may be covered by a very
thin (about 10 nm, for example) etch-stop layer 35 made of silica
or silicon nitride, for example. As shown in FIG. 3B, the a-Si:H
layer 32 is etched into a waveguide with tapered width W.sub.2, to
adiabatically transfer the light from the thick Si waveguide 33a to
the thin a-Si waveguide 32, located mainly on top of the Si
waveguide portion 33a. The bottom layer 34 is an insulating part of
a silicon substrate for the waveguide, and is here an SOI buried
oxide (BOX) layer. The BOX layer 34 has a lower refractive index
and a thickness that will optically separate the waveguide from a
higher-index silicon substrate (not shown), located underneath the
BOX layer.
[0055] The wide end of the tapered a-Si waveguide 32, having a
width of W.sub.3, is butt-coupled to an optical material that
comprises a photonic active device 31 deposited at the same height
as the waveguide 32 and having a comparable submicron thickness
t.sub.2. The joint and the device 31 is also shown in FIG. 3B. The
device 31 may be located on the Si layer 33, which forms a silicon
substrate and bottom cladding 33b for the device 31.
[0056] The device 31 can in some embodiments be made of the very
same amorphous silicon material as waveguide 32, e.g. as a
pn-implanted waveguide for phase modulation. In other embodiments
of the invention, it may be a waveguide grown on top of a
2-dimensional (2D) material like graphene, for example. In further
embodiments, the material may be a high refractive index material
like germanium (Ge) and the device may then be used as a detector,
for example, or a SiGe alloy in a Franz-Keldysh modulator, for
example.
[0057] The different widths W.sub.2 and W.sub.3 as a result from a
tapered shape of the waveguide in the horizontal plane of the
silicon substrate 34 are not the only way to increase the
cross-section of a thin waveguide 32. Alternatively, the shape may
be tapered in the vertical plane. The critical feature for the
waveguide is to have is to have a smaller volume in the area where
the light enters the waveguide and a bigger volume in the exit
area. The shape of the waveguide may be selected according to
various design criteria, and it need not to be linear and/or
planar, i.e. tapered as shown. Alternatively, the waveguide
cross-sections could be kept constant, whereas the refractive
indexes within a waveguide may vary, i.e. having a refractive index
gradient, to achieve similar adiabatic light transfer. Clearly a
combination of waveguide cross-section change and refractive index
change can be also used. The general criterion to efficiently move
the light from one waveguide to the other is to adiabatically
change from a condition where the effective index of the mode of
the thick waveguide (n.sub.eff1) is significantly higher than that
of the second waveguide (n.sub.eff2), to a condition where the
opposite is true (n.sub.eff2>n.sub.eff1). This can be achieved
by playing with the waveguide geometry (smaller waveguide
corresponding to lower effective index) and/or with the material
refractive index.
[0058] Depending on the type of device, the device can be either
coupled back to a further silicon waveguide 43 through a second
a-Si taper as shown in FIGS. 4A and 4B, e.g. for modulation, or it
can be just terminated as in FIGS. 3A and 3B, e.g. for Ge
detectors. The high refractive index layer 32 may consist of
hydrogenated amorphous silicon (a-Si:H), or of any transparent
material with refractive index higher than that of silicon, e.g. a
SiGe alloy.
[0059] In FIGS. 4A and 4B are shown a similar, but double-ended
device inventive photonic device 40 as in FIGS. 3A and 3B, with
thin double tapered waveguide 42a and 42b on each side of an
optically active material comprising a photonic active device 41,
all deposited on a silicon waveguide 43a, and the etch-stop layer
45. As in FIGS. 3A and 3B, the device 41 is located on an extension
of a generic thick Si substrate 43, forming a silicon substrate and
bottom cladding 43b for the device 41. Also a BOX layer 44 is
implied.
[0060] In FIGS. 5A and 5B is shown an embodiment of the present
invention, where a photonic circuit 50 has layers 51 of an optical
2D material, like graphene for example, which with intervening
dielectric layers 55, 56 constitute an optically active photonic
device. The submicron hydrogenated amorphous silicon (a-Si:H)
waveguide layer consists in this embodiment of three waveguide
portions 52a-52c deposited on top of a thick Si strip waveguide 53,
having three portions 53a-53c. Two thin waveguide portions 52a and
52b are tapered and deposited on the thick silicon waveguides 53a,
53b, as explained in connection with FIGS. 4A-4B. The third and
central thin waveguide portion 52c is formed as a square on top of
the photonic active device 51, 56 and a silicon cladding waveguide
portion 53c.
[0061] An advantageous feature of the configuration shown in FIGS.
5A-5B is that the optically active photonic device may be deposited
on the silicon waveguide 53a-53c prior to the deposition of layer
52a-52c, consisting of amorphous silicon for example. Thus the
optical device 51, 56 becomes embedded inside the optical mode,
which enables an easy and large overlap of the top waveguide
52a-52c compared to standard approaches, where the mode graphene
interacts mainly or only with the evanescent tail of the optical
mode, see FIGS. 3A and 4A.
[0062] Error! Reference source not found. shows for example how a
bilayer 51 of graphene can be embedded in the thin waveguide 52c,
sandwiched between three dielectric thin films 55 and 56,
consisting of e.g. SiO.sub.2, Si.sub.3N.sub.4, or Al.sub.2O.sub.3.
The large overlap with the graphene bilayer is obtained thanks to
the relatively low index contrast between amorphous and crystalline
silicon.
[0063] In some embodiments, the waveguide 52a-52c may be deposited
on the thick waveguide portion 53c so as to wholly or partially
overlap any photonic device, as shown best in FIG. 5B, which
photonic device may have a layered structure as described above, or
not. A BOX layer 54 is also implied, see discussion above.
[0064] In some embodiments, a high index contrast between amorphous
and crystalline silicon is wanted, for example when bends with
micron-scale bending radii are used to build micro-ring resonators
with a free-spectral range as large as possible. FIGS. 6A and 6B
shows an inventive photonic device 60 with high index contrast thin
waveguides 61, 62 located on a thick silicon waveguide 63a and the
cladding extension 63b of the Si substrate 63, respectively.
[0065] The waveguide 61 has a silica bottom cladding 67 formed in
the cladding 63b by selectively etching silicon away, and replace
it with silica 67. In this region the submicron waveguide 61 will
deposit direct on top of the silica cladding which leads to a high
index contrast waveguide suitable for tight bends.
[0066] Taken further, as shown in Error! Reference source not
found., it is possible to couple light into photonic circuits 78
fabricated on SOI wafers with submicron silicon layer that are
bonded directly onto an inventive photonic circuit 70 on top of a
thin amorphous silicon waveguide 72a-72c, having a silica bottom
cladding 77. In this fashion, two different device platforms may be
coupled at and around the marked area 71 through a suitable
combination of inverse tapers. Thus, light can be coupled back and
forth between devices with thick SOI waveguides 73a and devices 78
that are based on standard submicron silicon waveguide
technologies. A silicon substrate and cladding portion 73b and BOX
layers 74 are also indicated.
[0067] In FIG. 8 is shown a light beam 85 entering an inventive
device 80, like the one in FIGS. 7A and 7B. The light 85 enters at
one end of a thick SOI waveguide 83a at arrow A. As have been
described in connection with FIG. 2, the light escalates through
adiabatic transfer from the thick waveguide 83a to the thin
waveguide 82c, because, when wide enough, the effective refractive
index of the thin upper waveguide 82c becomes sufficiently high to
effect the adiabatic light transfer. The light 85 proceeds in the
waveguide 82c-82a, until it again escalates to a photonic circuit
88 in the area 81, which corresponds to the area 71 in FIG. 7B. As
in FIG. 7B, a silicon substrate and cladding portion 83b and a BOX
layer 84 is also indicated.
[0068] In the reverse direction, from a photonic circuit to
waveguides, modulated or otherwise processed light may be led out
from the photonic circuit by optical coupling to a submicron
waveguide, and further by adiabatic transfer to thicker
micron-scale silicon-on-insulator (SOI) waveguides.
[0069] It is to be understood that the embodiments of the invention
disclosed are not limited to the particular structures, process
steps, or materials disclosed herein, but are extended to
equivalents thereof as would be recognized by those ordinarily
skilled in the relevant arts. It should also be understood that
terminology employed herein is used for the purpose of describing
particular embodiments only and is not intended to be limiting.
[0070] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment.
[0071] As used herein, a plurality of items, structural elements,
compositional elements, and/or materials may be presented in a
common list for convenience. However, these lists should be
construed as though each member of the list is individually
identified as a separate and unique member. Thus, no individual
member of such list should be construed as a de facto equivalent of
any other member of the same list solely based on their
presentation in a common group without indications to the contrary.
In addition, various embodiments and example of the present
invention may be referred to herein along with alternatives for the
various components thereof. It is understood that such embodiments,
examples, and alternatives are not to be construed as de facto
equivalents of one another, but are to be considered as separate
and autonomous representations of the present invention.
[0072] Furthermore, the described features, structures or
characteristics may be combined in any suitable manner in one or
more embodiments. In the description numerous specific details are
provided to provide a thorough understanding of embodiments of the
invention. One skilled in the relevant art will recognize, however,
that the invention can be practiced without one or more of the
specific details, or with other methods, components, materials,
etc. In other instances, well-known structures, materials, or
operations are not shown or described in detail to avoid obscuring
aspects of the invention.
[0073] While the forgoing examples are illustrative of the
principles of the present invention in one or more particular
applications, it will be apparent to those of ordinary skill in the
art that numerous modifications in form, usage and details of
implementation can be made without the exercise of inventive
faculty, and without departing from the principles and concepts of
the invention. Accordingly, it is not intended that the invention
be limited, except as by the claims set forth below.
* * * * *